Full color organic electroluminescent (EL) devices, also known as organic light-emitting devices or OLED, have recently been demonstrated as a new type of flat panel display. In simplest form, an organic EL device is comprised of an electrode serving as the anode for hole injection, an electrode serving as the cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly assigned U.S. Pat. No. 4,356,429. In order to construct a pixelated display device such as is useful, for example, as a television, computer monitor, cell phone display or digital camera display, individual organic EL elements can be arranged as an array of pixels in a matrix pattern. OLED displays can be made to have one or more colors. These displays are known as multicolor displays. Full color OLED devices are also known in the art. Typical full color OLED devices are constructed of pixels that are red, green, and blue in color. That is, these pixels emit light in the red, green, and blue regions of the visible light spectrum. As such, the emitted light from the pixels would be perceived to be red, green, or blue by a viewer. These differently colored pixels are sometimes referred to as subpixels which taken together as a group form a single full-color-pixel. Full color organic electroluminescent (EL) devices have also recently been described that are constructed of pixels that are red, green, blue, and white in color. Such an arrangement is known as an RGBW design. An example of an RGBW device is disclosed in U.S. Patent Application Publication 2002/0186214 A1 now U.S. Pat. No. 7,012,588.
Several approaches to obtaining color displays are known in the art. For example, each differently colored pixel can be constructed using one or more different OLED materials. These materials are selectively placed on the differently colored pixels by methods including deposition through shadow masks, thermal transfer from a donor sheet, or ink jet printing. Another approach to producing a color display is to place OLED materials in a common stack of one or more layers across all the differently colored pixels, and then use one or more different color filters to selectively convert the common OLED color to different colors for each differently colored pixel. In this case the OLED materials are typically arranged so as to produce a broad emission spectrum, also referred to as white emission or white OLED. An example of a white OLED with color filters is disclosed in U.S. Pat. No. 6,392,340.
OLED devices having microcavity structures have been shown in the art. An example of an OLED microcavity device is shown in U.S. Pat. No. 5,847,506. In such a microcavity structure, light emitted by the OLED resonates between a reflector and a semitransparent reflector. The optical distance between the reflector and the semitransparent reflector can be adjusted to select the wavelength or wavelengths of light enhanced by the microcavity structure. Such a microcavity structure can yield highly efficient emission with a sharp emission spectrum that results in pure colors. It is possible to use such a microcavity structure in a device in which each differently colored pixel is constructed using one or more different OLED materials emitting in a certain region of the spectrum or, because of the microcavity structure's sharp output emission, to use it with broadband emitting materials that are common to all of the differently colored pixels, even in the absence of color filters applied to the differently colored pixels. In both cases, the optical length of the microcavity can be adjusted separately for each differently colored pixel in order to tune the output color of the differently colored pixel. Examples of OLED devices using broad emitting materials that have been coupled with a microcavity structure where the microcavity is separately adjusted for each differently colored pixel to produce a multicolor display are shown in U.S. Pat. Nos. 5,405,710 and 5,554,911.
Microcavity structures are commonly formed with highly transparent materials within the cavity between the reflector and semitransparent reflector. However, some materials that are commonly used in the construction of OLED devices are partially absorbing in the desired visible wavelengths. For example, OLED devices are frequently produced with a hole injecting layer to improve hole injection from the anode. One common hole injecting layer is CuPC (Copper (II) phthalocyanine) as described in U.S. Pat. No. 4,720,432. A hole injecting layer composed of a CuPC film followed by a thin fluorocarbon (CFx) film has also been described in the art. An example of such a CuPC device with a CFx film is discussed in U.S. Patent Application Publication 2004/0066139 A1 now U.S. Pat. No. 6,936,926.
CuPC is known to have significant absorbance for visible light of wavelengths greater than approximately 550 nm. In an OLED device that does not have a microcavity structure, this absorbance effect might be tolerable. However, because of the resonant nature of a microcavity structure, with light executing multiple reflections back and forth in the cavity, an OLED device constructed with a microcavity structure and having any partially absorbing materials within the cavity can have a substantial loss in light output efficiency due to the absorptions by these materials in the cavity. Therefore it is desired to have an optimized OLED device constructed with a microcavity structure that is tolerant of having an undesirably-absorbing material within the cavity wherein the loss due to the absorbance is reduced.